Allow trailing commas in function parameter lists

src/parsing/parser.cc

Index: src/parsing/parser.cc
diff --git a/src/parsing/parser.cc b/src/parsing/parser.cc
index 574472d2464ed646a752f194bfd8834ff3cbd314..7c2223f432ee8b57bae9680aa531ae505d91b388 100644
--- a/src/parsing/parser.cc
+++ b/src/parsing/parser.cc
@@ -816,6 +816,8 @@ Parser::Parser(ParseInfo* info)
       FLAG_harmony_exponentiation_operator);
   set_allow_harmony_async_await(FLAG_harmony_async_await);
   set_allow_harmony_restrictive_generators(FLAG_harmony_restrictive_generators);
+  set_allow_harmony_trailing_commas_in_parameters(
+      FLAG_harmony_trailing_commas_in_parameters);

[caitp (gmail), 2016/06/22 18:56:20]

You need to do something similar in Parser::ParseLazyFunctionBodyWithPreParser()
so the preparser has the same flags

   for (int feature = 0; feature < v8::Isolate::kUseCounterFeatureCount;
        ++feature) {
     use_counts_[feature] = 0;